Week 10: Medical applications of radiation

Question 1

What is the general method of X-ray diagnosis?

Answer 1

X-rays are attenuated by the body but the linear attenuation coefficient depends on tissue type.

Simple X-rays provide a 2D shadow of the patient.

This is known as projectional radiography.

Question 2

How can the range of body parts that can be imaged with X-rays be improved?

Answer 2

The use of contrast agents.

This approach relies on introducing material to the body that has a different X-ray attenuation to the surrounding tissue.

Question 3

What is a positive agent for X-ray contrast?

Answer 3

Have stronger absorption than most common elements in biological systems.

Question 4

Give two examples of positive agents.

Answer 4

Iodine (Z = 53)
Barium (Z - 56)

Question 5

What is a negative agent for X-ray contrast?

Answer 5

Air
Carbon Dioxide

Question 6

Give two examples of negative agents.

Answer 6

Carbon dioxide
Air

Question 7

What is X-ray CT dependent on?

Answer 7

The different absorption coefficients in different tissues in the body.

I(x) = I0 exp(-ux)

Question 8

What is the procedure of a CT scan?

Answer 8

The patient lies flat on a bed.

An X-ray tube rotates around the body.

X-ray transmission is measured on the opposite side of the body.

This is done for a set of angles.

The data is processed by a computer to build detailed 3D images.

Question 9

What is single photon emission computed tomography?

Answer 9

A gamma-emitting radioisotope is delivered to the particular organs of the patient by attaching the tracer to a radiopharmaceutical that targets a specific region.

Question 10

What radioisotope is usually used in SPECT?

Answer 10

99m Tc

Question 11

What is the procedure for SPECT?

Answer 11

The patient lies on a flat table.

A gamma camera rotates around the patient and collects gamma spectra at set intervals.

If the interval was a 5 degrees and the spectra was collected for 20s each, the total imaging time would be around 25 minutes.

Question 12

Why is Technetium-99m the most commonly used radioisotope for SPECT?

Answer 12

Single gamma transition

Good E-gamma for the application

Good half life

Question 13

Sketch a basic diagram of a SPECT detector?

Answer 13

(45)

Question 14

What is positron emission tomography used for?

Answer 14

PET can be used to produce 3D images of functional processes in the body.

Can be used to image changes in e.g. blood flow or metabolism.

Question 15

What is the procedure for PET?

Answer 15

A positron emitting radionuclide (known as a tracer) is introduced into the body on a biologically active molecule.

When a positron particle is emitted, it stops in 2-3 mm (which limits the resolution of this method) and annihilates with the emission of two back-to-back 511 keV photons.

Question 16

What is the most common nuclide used for PET imaging?

Answer 16

F-18

Question 17

What is the half-life of F-18?

Answer 17

109 minutes

Question 18

List three nuclides used for PET and state their half-lives.

Answer 18

F-18 (109 minutes)

C-11 (20 minutes)

N-13 (10 minutes)

Question 19

Why are the nuclides used for PET typically produced at a cyclotron on the same site as the imaging machine?

Answer 19

Due to their short half-lives.

Question 20

What is the simplest wat of irradiating a tumour?

Answer 20

Electron and photon therapy.

Delivers a high dose to the tissues of the way in and less on the way out de to the exponential absorption of energy.

Doses to healthy tissues that are between the tumour and the surface of the body can be reduced by the use of multiple (4 or 6) beams of radiation entering the body at different angles)

Question 21

What is fractionation?

Answer 21

The dose is spread out over time, allowing the healthy cells a chance to recover, which they are usually more able to do than cancerous cells.

Question 22

Why is fractionation used in treating cancer?

Answer 22

Used to deliver a higher dose to the tumour, while doing less damage to the healthy cells.

Question 23

What does the Bragg peak allow for in heavy charged particle therapy?

Answer 23

The Bragg peak gives a very high energy deposition in a small volume at the end of the particle range.

Careful choice of particle energy allows the range to be tailored to the depth of the tumour.

Question 24

What is the most common form of heavy charged particle therapy?

Answer 24

Proton beam therapy (PBT)

Question 25

What is the energy range of protons used in PBT?

Answer 25

50 - 250 MeV

Question 26

What are the advantages of proton beam therapy?

Answer 26

Deposits less energy in healthy tissues for a given dose to a tumour, allowing for the treatment of tumours surrounded by sensitive tissues. Can be used to deliver a higher dose to a tumour for a given dose to the surrounding tissue.

Question 27

What is Carbon-ion radiotherapy (CIRT)?

Answer 27

This approach is similar to proton beam therapy but replaces the protons with carbon atoms.

Question 28

Why is carbon-ion radiotherapy more advantageous for deep seated tumours?

Answer 28

Carbon ions have a higher LET than photons or protons. This treatment method is also less susceptible to variations in cell radio-sensitivity or oxygen level, due to the very damaging interactions of the ions.

Question 29

What is external beam radiotherapy?

Answer 29

Radiation therapy based on beams of radiation from outside the body

Question 30

What is boron neutron capture therapy (BNCT)?

Answer 30

This technique uses a two-stage process to deliver high LET radiation to a tumour. First the cancer is doped with boron using a suitable compound. Then a neutron beam is used to irradiate the tumour.

Question 31

What is brachytherapy?

Answer 31

A treatment that places a sealed source inside or next to the treatment region. Correct choice of the radionuclide and source allows the damage to be limited to the treatment area, while sparing nearby healthy tissues.

Question 32

What is radionuclide therapy?

Answer 32

Uses a radioactive source introduced into the body by injection or ingestion.

Question 33

Give an example of radionuclide therapy.

Answer 33

The use of I-131 to treat thyroid cancer. Iodine is taken up by the thyroid, so consuming sodium iodide containing I-131 allows dose to be delivered directly to the tumour site.

Question 34

List three typical radionuclide sources.

Answer 34

I-131 Lu-177 Y-90

Question 35

What is the idea of theranostics?

Answer 35

Where the same molecule us used for diagnosis and therapy. Tailoring the molecule to the patient gives a more personalised treatment, and minimises side effects from dose to healthy tissues.

Question 36

What are the two broad categories for the medical uses of radiation?

Answer 36

Diagnostic and therapeutic.

Question 37

List two positive contrast agents used in X-ray imaging.

Answer 37

Iodine Barium

Question 38

List two negative contrast agents used in X-ray imaging.

Answer 38

Air Carbon dioxide

Question 39

State the nuclide that decays to Tc - 99m and the nuclide Tc - 99m decays to, giving the half-life and the decay mode in both cases.

Answer 39

Mo-99 decays to Tc-99m by beta decay with a half-life of 2.75 days. Tc-99m decays to Tc-99 by gamma emission with a half-life of 6.01 hours.

Question 40

Explain why PET images have less noise than SPECT images.

Answer 40

PET scanners only count a detection if both of the back to back photons are detected, but SPECT has to rely on only one photon. This means that PET scanners can discount noise more easily than SPECT scanners.

Question 41

What molecule is most commonly used for PET scans and how does the body treat this molecule?

Answer 41

Fluorodeoxyglucose (FDG) is most commonly used for PET. The body treats it like glucose.

Question 42

Briefly explain how multiple photon beams can be used to lower the damage to healthy tissue in radiotherapy.

Answer 42

Using multiple photon beams that intersect on the tumour site allows the same dose to be delivered to the tumour while spreading the dose to healthy tissue over a larger region (and therefore mass, lowering the dose) meaning less damage is caused.

Question 43

In radiotherapy, what is meant by the term "fractionation"?

Answer 43

Splitting the total dose into smaller parts that are delivered over a period of time to allow an overall higher dose.

Question 44

Briefly explain the purpose of the spread out Bragg peak.

Answer 44

The spread out Bragg peak is used to deliver dose across a finite region, as the single energy Bragg peak is very narrow and unlikely to cover the whole depth of a tumour.

Question 45

State the nuclear reaction used in boron neutron capture therapy.

Answer 45

n + 10B ---> 7Li + alpha

Question 46

What is the difference between brachytherapy and radionuclide therapy?

Answer 46

Brachytherapy uses sealed sources, while radionuclide therapy uses unsealed sources that allow the radionuclide to interact with the body's physical processes.